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Aqueous Absorption of NOx
Induced by Sodium Chlorite Oxidation
BY
Chen-Lu Yang
Thesis submitted to the Faculty of the Graduate School of the New
Jersey Institute of Technology in partial fulfillment of the
requirements for the degree of Master of Science in
Environmental Science. 1944-9
APPROVAL SHEET
Title of thesis : Aqueous Absorption of NO x Induced by Sodium
Chlorite Oxidation
Name of candidate : Chen-Lu Yang
Master of Science in Environmental Science,
19B9
Thesis and abstract approved
Dr. Henry Shaw
Professor of Chemical Engineering
Date approved:
Dr. Joseph W. Bozzelli Date
Dr. Richard Trattner Date
VITA
Name : Chen-Lu Yang
Degree and date to be conferred : M.S. in Environmental Science,
1989
Secondary education : Makong Provincial High School
College attended Date Degree Date of degree
Feng-Chia University 80-84 B.S. 1984
NJIT 87-89 M.S. 1989
Major : Environmental Science
Position held : Research Assistant of the Gas Scrubbing Laboratory
at NJIT, Newark, N.J.
ABSTRACT
Title of thesis : Aqueous Absorption of NO x Induced by Sodium
Chlorite Oxidation
Chen-Lu Yang : Master of science in environmental science,
1989
Thesis directed, by: Dr. Henry Shaw
Professor of Chemical Engineering
Aqueous scrubbing in alkaline media for the removal of
nitrogen oxides (NOx) from flue gases using NaClO2 as an oxidant
is being investigated in the Chemical Engineering Department of
New Jersey Institute of Technology. This thesis presents the
findings of an experimental program that was conducted to
estimate the capability of NaClO2 aqueous solution to oxidize
insoluble NO to soluble NO 2 in order to increase its solubility
and thus prevent NO x emission from entering the atmosphere from
flue gases emitted by municipal incinerators or power plants.
The key findings of this program are:
* NO is not absorbed by NaOH aqueous scrubbing solutions.
* NO2 is essentially quantitatively absorbed in NaOH aqueous
scrubbing solutions.
* Addition of NO 2 improves NO absorption in NaOH solution.
* NaClO2 containing aqueous scrubbing medium quantitatively
oxidizes NO to nitrate.
* Concentrations greater than 0.1M NaOH inhibit NO
absorption in 0.008M NaClO2 containing aqueous scrubbing
media.
* Aqueous scrubbing media containing less than 0.1M NaOH
quantitatively absorb NO induced by NaClO 2 oxidation.
* The presence of 5% Oxygen in flue gases reduces by 11.5% NO
absorption induced by NaClO 2 oxidation.
* Mass transfer rather than chemical reaction kinetics seems
to dominate the NO absorption process.
ACKNOWLEDGEMENT
The author is most grateful to his advisor, Dr. Henry Shaw, who
has contributed in so many ways to make this research a great
success.
My special thanks go to Dr. Joseph W. Bozzelli and Dr. Richard
Trattner, for their thoughtful advice , helpful suggestions and
ideas in conducting this research.
The author acknowledges with much appreciation the direct and
indirect assistance by his colleague Joseph Chou.
TABLE OF CONTENTS
ABSTRACT
1. INTRODUCTION 1
1-1 NOx Formation and Sources 1
1-2 Health Effect of NO
x
5
1-3 Legislation 6
1-4 Technologies for NO x Control 7
2. GAS ABSORPTION 11
2-1 General Concepts 11
2-2 Physical Absorption 13
2-3 Chemical Absorption 14
2-4 Controling Factor Determination 18
2-4-1 Physical Absorption Controlling
Factor Determination 18
2-4-2 Chemical Absorption Controlling
Factor Determination 19
3. A LITERATURE REVIEW OF NO ABSORPTION 22
3-1 Equilibrium 22
3-1-1 Gas Phase Equilibrium 22
3-1-2 Aqueous Phase Equilibrium 24
3-1-3 Solubility 25
3-2 Absorption of NO Followed by Oxidation 27
3-3 Absorption of NO Followed by Complex Reactions . 31
4. EXPERIMENTAL PROGRAM 33
4-1 Apparatus 33
4-1-1 Flue Gas Blending System
4-1-2 Scrubbing Vessels
33
34
4-2 Reagents 36
4-2-1 Sodium Chlorite 36
4-2-2 Sodium Hydroxide 37
4-2-2 The Gases 37
4-3 Analysis 40
4-3-1 Flue Gas Analysis 41
4-3-2 Scrubbing Solution Analysis 43
5. RESULTS .... 48
5-1 Absorption of NO in Caustic Aqueous Scrubbing .. 49
5-2 Absorption of NO 2 in Caustic Aqueous Scrubbing . 51
5-3 NO2 Effects on NO Absorption in Caustic Aqueous
Scrubbing 53
5-4 Oxygen Effects on NO Absorption in Caustic
Aqueous Scrubbing 56
5-5 Oxidation of NO by NaClO2 in Aqueous Scrubbing . 58
5-6 Oxidation of NO by NaClO2 in Caustic
Aqueous Scrubbing 65
5-7 Oxygen Effects on NO Absorption in NaClO 2
5-8
Aqueous Scrubbing
pH Effects on NO Absorption in NaClO2
67
Aqueous Scrubbing 69
6. DISCUSSION 72
7. CONCLUSIONS AND RECOMMENDATIONS 75
7-1 Conclusions 75
7-2 Recommendations for Future Work 76
REFERENCES 78
1. INTRODUCTION
Nitrogen oxides (NOx) are mixtures of compounds of nitrogen
and oxygen, namely, N 2O, NO, N 2O 3 , NO 2 , N204 and N2O 5 generally
found in effluents from combustionsources. The predominant NO x
compounds are NO and NO 2 . They are formed as a result of the
reactions of atmospheric nitrogen with atmospheric oxygen at high
temperatures. Also, large quantitative of NO x are formed from the
oxidation of nitrogen compound found in fuel or in wastes being
incinerated. More than 90% of NOx emitted from stationary
combustion sources consist of NO which is relatively insoluble in
inorganic aqueous solution. finding a way to control NO is
essential for the prevention of NO x pollution.
The molecular and thermodynamic properties of NO according to
Yost and Russel (1944) are sumarized in Table 1 . Its bonding
type can be studied by magnetic measurements, by which the
participation of the odd electron in the bond is probed, or by
measuring the change in the bond strength between nitrogen and
oxygen atoms, as manifested by its infrared spectra. The
configuration of NO molecular orbitals and the bonding of the
nitric oxide group is illustrated in Figure 1 (M. Shelef 1971).
1-1 NOx formation and sources
Nitrogen oxides are emitted as nitric oxide (NO) from a
variety of sources. The estimated emissions of NO x have large
uncertainties associated with them. About 70-90% of Atmospheric
NOx emission are from anthropogenic sources in the world (Singh
1987).
Molecular Properties
Bond energy 151 Kcal/Mole
Thermodynamic Propertiesm.p.
109.4945K
Bond length
1st ionization potential
o1.15 A
9.5 V
ΔH fusion
b.p.
549.5 cal/mole
121,36°K
Dipole moment 0.16 Debye pH vaporization 3293 col/mole
Molal susceptibility, 20 °C 1.43.10-3 cgs unitsΔH°f,298K +21,500 cal/mole
Ground state 2π 1/2 ΔF°f,298K +20,650 cal/mole
1st excited state 2n3/ 2 ΔS°f,298K 50.35 e. u.
ΔE(2" 1/Z -' 213/2) 352 cal/mole (124.2 cm 1)
Figure 1 Bonding and Electron configuration of NO
1-1-1 Fossil fuel combustion and biomass burning
Thermally generated NO results from the reactions of oxygen-
nitrogen mixture exposed to a high temperature. The maximum
concentration of NO can be estimated from the equilibrium
composition of an oxygen-nitrogen mixtures as a function of
temperature (Shelef, 1971). The equilibrium compositions of NO as
a function of temperature are summarized in Table 2.
The key chemical reactions in the oxygen-nitrogen mixture at
temperature below 4,500 K are given in Table 3. Dissociation of
N 2 into atomic nitrogen becomes important above 4,500K.
(Seinfeld, 1986).
1-1-2 Lightning
The major natural source of NO is caused by the effect of
atmospheric lightning on atmosphere oxygen and nitrogen. The
temperature of a column of air exposed to atmospheric lightning
is a function of energy deposition of the lightning. For a
typical energy deposition of about 10 5 J/m, the column of
air immediately surrounding the charge is heated to temperature
on the order of 30,000K. At temperature above 2,300K, NO is in
thermodynamic equilibrium with N 2 and O 2 . As the temperature of
the heated air drops below 2,000K, NO "freezes " as a stable
compound.
The total global production of NO due to atmospheric
Table 2 Equilibrium Concentration of NO in. Nitrogen-Oxygen
Mixture
TemperatureK K
Equilibrium NOConcentration, Dom
298 1.549 x 10 13 5.6 x 10-11
400 1.413 x 10 11 6.15 x 10-7
500 6.223 x 108 1.39 x 107 4
600 1.663 x 107 5.2 x 10.-3
700 1.268 x 10 8 6.85 x10- 2
800 1.828 x 10 3 4.75 x 10-1
900 4.046 x 10 4 2.14
1,000 1.208 x 10 4 7.23
1,100 4.508 x 10 3 19.3
1,200 1.977 x 10 3 43.5
1,300 9.863 x 10 2 88.0
1,400 5.433 x 10 2 160.0
1,500 3.236 x 10 2 269.0
1,600 2.056 x 10 2 424.0
1,700 1.380 x 10 2 630.0
1,800 9.683 x 10 1 905.0
1,900 7.047 x 10 1 1230.2
2,000 5.297 x 10 1 1640.0
Table 3 Key Reactions of Nitrogen-Oxygen Mixture at High
Temperature
ReactionForward Reaction
cm3 molecule -1 sec -1 dReverse Reaction
cm3 molecule -1 sec-1
O2 + M ←→ 20 + MC 1.876 X 10 -6T -1/2exp(-118,000/RT) 2.6 X 10 33E'
O + N2 ←→ NO + N 1.16 x 10 -10exp(-75,500/RT) 2.57 x 10 -11
N + O2 NO + 0 2.21 X 10-14 Texp(-7080/RT) 5.3 X 10 -15 TX exp(-39,100/R T)
lightning is the product of two terms: 1) The yield of NO per
energy input or lightning (J) or the NO per lightning flash and
2) the total energy deposited by lightning or the lightning flash
frequency (Levine, 1984).
1-1-3 Microbial activity in soils
It has been found from field flux measurements and laboratory
experiments that soil microrganisms may also produce appreciable
amounts of NO. To quantify the rate of production of NO, a
series of laboratory experiments were performed using two of the
most common bacteria found in soils, Nitrosomonas Europaea and
Alcaligenes Faecalis. The results indicate that nitrification is
an aerobic process in which ammonium is oxidized to nitrite and
nitrate. NO and N 20 are direct or indirect intermediates of this
pathway (Levine, 1984).
1-1-4 Diffusion
NOx diffusion from the stratosphere is also an important
source of tropospheric NON .
1-2 Health effects of NOx
Little data exists on the health effects of NON because
exposure to these compounds is usually accompanied by
concurrent 'exposure to other pollutants. Previous studies have
demonstrated that NO 2 causes emphysema in several animal species.
EPA's "Cincinnati dog study" which used 0.64 ppm NO 2 in
combination with 0.25 ppm NO, also indicated that the effect was
progressive. The airway resistance of the lungs is known to
increase when moderate exposures to NO x occurs. The principal
epidemiological study of the effects NO 2 was conducted on the
population of Chattanooga, Tenessee, in 1978, where manufacturing
operations produced an atmosphere contaminated primarily by this
compound. A conclusion from this study is that respiratory
illness aer higher than normal, if the concentration of NO 2
exceeds 0.06 ppm This study has been criticized for several
reasons, but it remains the main source of data on the health
effects of NO2'
1-3 Legislation
Man-made NO emissions frequently accompany combustion.
Various emission standards, limits and regulations have been
promulgated to control NO x emission. In the USA, the primary
standard for nitrogen dioxide is 10Oug/m 3 , taken as an annual
arithmetic mean. But there is no standard for nitric oxide. Table
4 compare the emission standards of Sitzerland, Austria,
Netherland, the FRG and Japan with the USA. The New Source
Performance Standards for large utilities requires NO x emissions
not to exceed 86g/EJ for gaseous fuels, 130g/EJ for liquid fuels
and 260g/EJ for solid fuels (taken as NO2).
Tble 4. Emission Standards for New Power Plants
Fuel (g MJ -1 )State power (MW) solid liquid gas
Switzerland > 300 0.07 0.04 0.03
50-300 0.14 0.07 0.05
1-50 0.18 0.07 0.05
Austria > 300 0.05 0.03 0.02-Draft 150-300 0.11 0.06 0.04
50-150 0.11 0.08 0.05
Netherlands > 300 0.14 0.08 0.05
50-300 0.18 0.08 0.05FRG > 300 0.07 0.04 0.03
50-300 0.14 0.08 0.05
1-50 0.18 0.13 0.05
Japan > 550 0.14 0.07 0.03
400-550 0.18 0.07 0.03
30-400 0.18 0.09 0.06
5-30 0.25 0.09 0.07< 5 0.25 0.10 0.08
USA > 73 0.28 0.14 0.09
29-73 0.32 0.14 0.05
1-4 Technologies for NOx control
Emission of NOx has become a serious environmental problem
and it is necessary for existing power plants plus municipal
and industrial incinerators, utilizing high temperature
processes, to remove the NO x from flue gases in order to comply
with federal and local regulations.
In the past few years, a variety of processes have been
proposed to eliminate this problem. These processes can be
summarized as follows:
A). Combustion modification
* Low NOx burner design: Various burner designs have been
proposed to lower the NO R emission such as
tangentially fired boiler. Traditional cyclone burners
produced high concentration of NOR in flue gas. Because
the temperature of heat sinks of tangential firing
burner almost equals to the temperature of burner
flame, it reduces 50% to 60% thermal NO R formation
(Maulbetsch, 1986).
* Flue gas recirculation: Flue gas recirculation is an
effective method to reduce thermal NO R . The
recirculated flue gas lowers the flame temperature and
the oxygen contain in combustion. According to the
report of EXXON research, 50% of flue gas
recirculation reduces 60% thermal NO R formation
(Sommerlad, 1971) , (Shaw, 1981).
* Air-fuel staging in the combustion chamber: Thermal
NOx formation can be reduced by operating the
combustion in a fuel rich mode, followed by interstage
cooling and burnout of the uncombusted compounds with
excess air. Incomplete combustion in the first stage
lowers thermal NO formation. The second stage uses
excess air to complete the combustion, reduce the
combustion temperature and lower the thermal NO R
formation (Bienstock, 1972). Alternatively, another
method of interstage cooling can be used in order to
avoid a large excess of stoichiometic air.
B). Flue gas cleaning
* Selective noncatalytic reduction: NO can be reduced
to N2 and H20 by introduce NH 3 into the effluent gas
from a combustor at 930 to 990 °C. The majordisadvantages of this process are the narrow
temperature window, the need for well mixed NH 3 and the
possibility that NH 3 may break through (Seinfeld, 1988).
* Selective catalytic reduction: This process operates
similiary to the thermal process• except with a
catalyst. The reactions occur at a much lower
temperature of 300 to 400 °C. There are commercialcatalyst containing noble metals or vanadia/titania
that can control NO by 90% for the former, and 80% by
the latter. Much research is currently being conducted
for using Zeolites to control NO with NH 3 (personal
communication with Dr. Henry Shaw).
* Adsorption by solids: A number of activated carbon
processes have been proposed to remove NO from flue
gas. Copper oxide is also an effective adsorbent.
Using these processes, Technologies of adsorbent
regeneration and SO 2 removal are essential (Seinfeld,
1988).
* Wet process: In these processes, solvents are used to
scrub NO from flue gases. chemical reaction is required
to absorb NO in water. Those chemical reactions can be
categorized to Gas Phase oxidation, liquid phase
oxidation and, chelating reduction.
The gas scrubbing research presented here was conducted in
the Gas Scrubbing Laboratory (room 206 Advanced Technology
Center) of NJIT since May 1988. It is categorized as liquid phase
oxidation, In this process, NO x is absorbed into alkaline
solution and oxidized to nitrate.
2. GAS ABSORPTION
2-1 General concepts
Absorption is a diffusional process that involves the
transfer molecules from the gas phase into the liquid phase
because of the concentration gradient between the two phases. The
removal of contaminant by gas absorption occurs by bringing
the gas into contact with a liquid in which the contaminant alone
are soluble. Absorption of any species occurs at the surface
of the liquid film (e.g., surrounding the packing (or mesh) or at
the bubble surface when the gas is the dispersed phase).
Absorption is a unit operation that is enhanced by all the
factors which normally affect mass transfer. There are high
interfacial area, turbulent contact between the two phases,
increased residence time, repetive contact, high solubility, high
diffusion coefficient, low liquid viscosity, and large negative
Gibb's free energy changes in the case of reactive scrubbing.
These factors influence the rate of mass transfer because they
directly influence the concentration gradient.
The two film theory of gas absorption is the basic model of
describing mass transfer from a gas to a liquid. The model is
based on the theory that thin films exist on both sides of the
phase boundary between gas and liquid. Molecular mass transfer or
diffusion can only occur through these films. Figure 2 shows the
interface behavior of two film theory for the complete range of
11
the reaction rate and mass transfer rate.
Figure 2. Interface Behavoir of Two Film Theory
Case A,B,C and D: Reactions are so fast that component A is
consumed before it diffuse into the main body of liquid. Reaction
occurs only in the liquid film.
Case E and F: Reaction rates are intermediate with respect to
mass transfer. Reaction will occur in both film and main body of
the liquid.
Case G: Reaction rate is slow with respect to mass transfer.
Reaction occurs in bulk liquid, however the film still provides a
resistance of component A to transfer to the bulk liquid.
Case H: Reaction rate is infinitively slow with respect to
mass transfer. Equilibrium is established and absorption is
controled by chemical reaction only.
2-2 Physical absorption
The amount of a gas being physically absorbed in a liquid is
determined by the solubility of the gas in the liquid. Solubility
can be expressed in terms of partial pressure through Henry's Law
as
Pa = Ha*Xa
Where Ha : Henry's Law constant of A
Pa : partial pressure of A
Xa : mole fraction of A in liquid phase
The mass transfer rate is the product between the mass
transfer coefficient and the concentration difference between the
13
bulk and the phase boundary.
Na = Ka*(Pag-Pa1 )
In this case we may lump Henry's Law constant into the relation
between overall mass transfer coefficient, K a , gas phase mass
transfer coefficient, Kag and liquid phase mass transfer
coefficient, Kai
1 1 Ha
K a Kag Kal
Henry's Law is obeyed, when
1) The concentration of disolved gas is very low.
2) The temperature and pressure are far away from the
critical temperature and pressure of the gas, and
3) The gas does not change it's molecular structure when it
disolves (Bethea, 1978).
Physical absorption models are not suitable for reactive
scrubbing process.
2-3 Chemical absorption
When a scrubbing solution contains a compound B, which can
react with A, the contaminant in gas phase, it is refer as
chemical absorption. The compound will prevent the concentration
of A to build up in the bulk liquid. The overall rate expression
for the reaction will have to account for the mass transfer
14
resistance (to bring reactants together) and the resistance of
the chemical reaction step. Since the relative magnitude of these
two resistances can vary greatly, we have a whole spectrum of
possibilities, and since each situation requires its own
analysis. our first problem is to identify these kinetic regimes
and to select the one which matches the given physical situation.
Depending on the relative rates of diffusion and reaction the
absorption systems are classified into five different regions
(Levenspiel, 1972).
* Case A: Instantaneous reaction with respect to mass
transfer. Consider an infinitely fast reaction of any order:
+ bB > productaA (from gas) (liquid)
We have
If Ca is not too high, We have the situation shown in Figure 3.
* Case B: For fast second order reaction rate between A and
B, the rate expression can be written as follows:
15
Figure 3. Concentrations of Reactants for An Infinitely Fast
Irreversible Reaction (Levenspiel, 1972)
* Case C: Intermediate Rate with respect to mass transfer.
These cases represent the general situation where reaction occurs
both within thin film and within the main body of liquid. The
general rate expressions for this regime have not been
determined.
For our purposes, it suffice to note that the rate is
16
dependant on both interfacial surface and on volume of liquid,
thus the ratio
Where s is interfacial surface
v is the volume of liquid
becomes a parameter in this regime.
* Case D: Slow reaction with respect to mass transfer. Here
the two films and main body of liquid act as resistances in
series, thus we may combining and eliminating intermediate
concentrations and produce the rate expression as follows
(Levenspiel, 1972):
* Case E: Infinitely slow reaction: Here the mass transfer
resistance is negligible, the composition of A and B are uniform
in the liquid, and the rate is determined by chemical kinetics
only. The rate can be given directly by
17
Usually cases A and B describe gas scrubbing process for air
pollution control.
2-4 Controlling factor determination
Absorption is classified into physical and chemical
absorptions. Since there is no reaction in physical absorption,
it is controlled by solubility while either chemical reaction or
diffusion may control a chemical absorption.
2-4-1 Physical absorption controlling factor
determination
The value of Henry's Law constant and the relation between
the overall, gas phase and liquid phase mass transfer
coefficients can be used to determine which phase controls the
rate of mass transfer in a physical absorption process.
The gas phase resistance is controlling when the gas is
highly soluble. This means that there is virtually no resistance
in absorbing the gas once it reaches the liquid interface. A high
solubility also implies a high diffusion rate in the liquid
phase. Therefore the rate controlling step must either be gas
18
phase diffusion or the resistance associated with crossing the
interface. Since the two film theory assume that there is no
resistance at the interface, the gas phase diffusion is the
controlling factor in this case.
The opposite situation occurs when one component has limited
solubility in the liquid phase. There is resistance to
disolution. The diffusion in the liquid will be low. As the gas
phase maintains a nearly saturated interface, the mass transfer
rate depends on the rate at which the solute can be removed from
the interface. In this case, the local and liquid film
coefficients for mass transfer are approximately the same.
If the value of the Henry's law constant is < 0.187 atm.
l/g.mole, the gas phase controls the rate of mass transfer. If
H > 187.3 atm.1/g.mole, the liquid phase is controlling.
Another way to determine which phase offers the most
resistance is to examine the ratio of the local and overall mass
transfer coefficients. If K g/Kog < 2, the gas phase probably
controls. If this is < 1.1, the gas phase definitely controls. If
the ratio is high,about 10, the liquid phase is controlling
(Bethea, 1978).
As a general rule, gas phase diffusion is much faster than
liquid phase diffusion and thus governs the type and size of the
absorption equipment selected for any particular application.
2-4-2 Chemical absorption controlling factor
determination
19
To determine whether a reaction is fast or slow, we focus on
unit surface of gas-liquid interface. we assume that gas phase
resistance is negligible and we define a film conversion
parameter (Levenspiel, 1972):
If M >>1 all reaction occurs in the film, and surface area is
the rate controlling factor. On the other hand, if M << 1, no
reaction occurs in the film, and bulk volume becomes the rate
controlling factor. More precisely, it has been found that:
A) If M > 4, reaction occurs in the film and we have cases A
and B in previous section.
B) If 0.0004 < M < 4, we then have the intermediate cases, C
and D.
C) If M< 0.0004, we have the infinitely slow reaction of
case E.
For process design, when M is large, a contecting device
is chosen which develops or creates large interfacial area,
energy for agitation is usually an important consideration. On
20
the other hand, If M is very small, all we need is a large volume
of liquid.
3 A LITERATURE REVIEW OF NO ABSORPTION
The removal of NO from flue gases has received considerable
attention due to federal regulations associated with the "Clean
Air Act". the absorption of NO from flue gases is very complex
because:
a) The NO gas is a mixture of several components consisting
primarily of NO and NO 2 but could contain trace amounts of N 20
N2O3 N2O4 and N2 O 5.
b) More than forty equilibrium reactions may be needed to
explain the chemistry between nitrogen oxides and oxyacids.
c) Absorption and desorption operations occur simultaneously.
d) All kinds of complex reactions occur in this system.
e) Needed physical-chemical data is generally not available.
During the past fifty years, much data have been published
in various aspects of NO absorption. Much of these data were
compiled in order to define areas for the future research.
3-1 Equilibrium
In the absorption of NOx gas in aqueous solutions, every
component in the system takes part in one or more reaction with
other components. Equilibria occur in the gas as well as in
the liquid phase. In the following section, this type of
information will be reviewed.
3-1-1 Gas phase equilibrium
22
The following equilibria occurs in the gas phase:
K 12 NO 2 (g) <----> N 2 O 4 (g)
K2NO(g) + NO 2 (g)< ----> N2O 3 (g)
K3NO(g) + NO 2 (g) + H2O(g) <----> 2 HNO 2 (g)
Where K 1 , K 2 and K 3 are the pressure based equilibrium
constants of each reaction. The K 1 value is given by the
following equation (Bronsted, 1922; JANAF 1971):
2993log K1 = 11.232
T
In SI unit (kPa) -1 . and T in K.
The K2 value is given by (Beattie, 1963; Beattie and Bell,
1947):
2072log K2 = 9.240 (kPa) -1
T
The value of K3 reported by Wayne and Yost (1951); Waldorf
and Babb (1964); Ashmore and Tyler (1961) and Karavev (1962) is
1.4*10 -2 . at 25 °C. However, the NBS (1968) and JANAF (1971)
compilations give the values of 1.7 and 0.194 kPa -1 respectively.
Schwartz and White (1981) analyzed the published literature and
they compiled the data on standard free energies of formation
summarized in Table 5.
Table 5 Standard Free Energies and Enthalpies of Formation
at 298 K
Species
Standard Free Energyof Formation
kJ/kmole x 10 -3
Standard Enthalpyof Formation
kJ/kmole x 10 -3
NO2 51.32 33.19NO 86.61 90.29N
2
O3 139.48 83.76N
2
O4 97.87 9.17N 2O 5 115.12 11.30HNO 3 -74.18 135.12HNO 2 -46.05 -79.53H 2O(g) -228.70 -241.934H 2O(1) -237.292 -285.967
3-1-2 Aqueous phase equilibria
Aqueous phase equilibria can be written in terms of gas phase
and mixed phase equilibria. Consider the following reactions:
K42 NO2 (g) + H2O (l) <----> HNO 2 (aq) + NO 3 (aq) + H+ (aq)
K5NO(g) + NO 2 (g)+ H2O <----> 2 HNO 2 (aq)
K62 NO(g) <----> 2 NO(aq)
These equations give:
Kal3 HNO2(aq) <----> H+ (aq) + NO 3 -(aq) + 2 NO(aq) + H 2O(l)
24
K4 Where Kai HNO
2
(k5 )
Ka22 NO <--- <----> 2 H+ + NO + NO -3
K4Where ka2 =
HNO22
Schwartz and White (1981) reviewed the experimental data
on liquid phase equilibrium. The result are summarized in Table
6. The symbol w in equations indicates that water take part in
these reactions.
3-1-3 Solubility
(A) Nitric oxide (NO)
Solubility of nitric oxide in water with respect to
temperature has been reported in International Critical Tables
Table 6 Aqueous phase equilibria
No. Equilibrium Value
A, 3HNO2(1) ---2-2=-= H+ + NO3 + 2N00) 1.12 x 10' (kmole/m")
A 3 2H' + 3HNO -.=-1-1:-6-NO; + 2N0(1) 8.46 x 10' (kmole/m 3 ) -2
A, 2NO2(1)--= N 2 04 (1) 6.54 x 104 (kmole/m 3 ) - '
/13 NO(1) + NO 2 (1) ;-----• N 2 0 3 (1) 1.37 x 104 (kmole/m 3 ) - '
A6 2NO2(1)4:-! -:.-- HNO2(l) + fl" + NO3 3.8 x 10 9 kmole/m 3
A, N203(1) :_2--.L. 2HNO 2 (1) 3.3 x 10 2 kmole/m 3
AB NO(1) + NO 2 (1)`.---7-4:- .- .'2FINO 2 (1) 4.52 x 10 6 —
' The symbol indicates that water takes part in the reaction.
25
(1928), Handbook of Chemistry and Physics (1963) and by Seidell
and Linke (1965). The value of Henry's law constant can be
correlated by the following equation
-1463.32log HNO = + 2.178
T
Where HNO is in kg mole/m 3 atm.
Armor (1974) has measured the solubility of NO as a function
of pH (2-13). He has shown that, in the pH range studied, there
was no change in the value of the solubility.
The solubility of NO in aqueous electrolyte solutions can be
calculated by using the following equation
Hlog ( ) = -Σ Ks * I
Hw
Where H and Hw are the values of solubility in the solution
and water, respectively. K s is the salting out parameter and is
given by the following equation
Ks = i+ + i- + ig
Where i+ , i - are the contributions of the cations and
anions of the electrolyte and ig is the contribution of the
solute gas. Danckwerts (1970) reported the values of i + and
i- for various cations and anions. On the basis of these values
and the solubility data reported by Armor (1974) the value of i g
26
for NO is 0.037 m 3/k ion.
(B) Nitrogen dioxide (NO 2 )
The determination of solubility of NO 2 in water is difficult
because NO2 reacts with water. Andrew and Hanson (1961) and
Komigama and Inoue (1980) found the solubility of NO 2 in water
using the theory of absorption followed by pseudo nth order
chemical reaction. The value of Henry's law constant was found to
be 0.04 at 25 °C [Andrew and Hanson] and 0.024 at 15 °C [Komiyama
and Inoue]. This method needs the knowledge of diffusivity (D) of
NO2 in water and the reaction rate constant (k) between disolved
NO2 and water.
Garcdel et al., (1975) obtained a preliminary estimate of
Henry's law constant of NO 2 on the basis of following equation
HNO2 HCO2 =
HNO HCO
The value HNO2 works out to be 0.07 (kgmole/ m3 atm) at 25°C.
Schwartz and White (1981) have correlated the solubilities of
gases with their boiling points. Using this relation the value of
HNO2 = 0.02 was estimated at 25 °C.
3-2 Absorption of NO followed by oxidation
Oxidation of NO is the most important step in the NO
scrubbing process. Oxidation of NO to NO 2 is relatively slow, and
thus, becomes the rate limiting step in most approaches. Further
27
more, chemical oxidation of NO is required when its concentration
is very small. The common agents include; nitric acid, ozone,
hydrogen peroxide, chlorine dioxide, chlorite, etc.
(A) Ozone
The reaction of NO with ozone is given by the following
equation
NO + O3 ----> NO2 + O 2
The resulting nitrogen dioxide can conveniently be scrubbed
in caustic solutions even at low ppm values. The mechanism of
above reaction has been studied by Harcourt (1972) and Bhatia and
Hall (1980). The kinetics has been investigated by Borders
(1982).
(B) Sulfuric acid
Topol, et al., (1968) used sulfuric acid as the oxidizing
agent. The reaction is represents by:
4 NO + 3 H2SO4 <----> 2 NO+ +2 NO2 + H 3O
+ + 3 HSO
2 NO + 5 H2SO4 <----> 2 NO+ + SO2 + 2 H 3 O
+ + 4 HSO 4
The equilibrium constant values of the first and second
reactions were found to be 2*10 50 (kgmole/m 3 ) 3 and 4*10 12
(kgmole/m 3 ) 7 , respectively. The rate of the second reaction was
second order in NO and the rate constant was found to be 0.2
(m3/kgmole s).
(C) Hydrogen peroxide
28
Takahashi et al., (1979) carried out gas phase NO oxidation
using hydrogen peroxide. Nitric oxide and hydrogen peroxide were
taken in the ratio of 1 : 13 at a temperature more than 400 ° C
and using 100 ppm NO. The extent of NO oxidation was greater than
95%. Absorption experiments were carried out using 101.5 mm ID
stirred cell by Lodhabhoy and Sharma (1969). The reaction between
NO and H2 O2 is given by the following equation
NO + H2 O2 <----> NO2 + H 2 O
Absorption of NO is accompanied by fast pseudo first order
chemical reaction. The value of reaction rate constant at 30 °C
was found to be 8.42*102 (m 3/kgmole s) with an activation
energy of 13,700 kcal/kg mole.
(D) Potassium permanganate
Potassium permanganate was used for the oxidation of NO by
Kann et al., (1976). Conversion of NO exceeding 80% has been
reported. Sada et al., (1977) studied the oxidation of NO using
aqueous solutions of KMnO 4 and aqueous solutions mixed solutions
of KMnO 4 and NaOH.
In the absence of NaOH the reaction between NO and KMnO 4 can
be represented by the following equation:
NO + MnO4 ----> NO 3 MnO 2 (s)
Uchida et al., (1983) showed that the product of reaction with
MnO 2 has some inhibiting effect on the rate of reaction.
29
In the presence of NaOH, the oxidation reaction enhances the
reaction between NO and KMnO 4 in a strong aqueous caustic
solution is given by
NO + MnO4 + 2 OH ----> NO2 + MnO 4-2 + H2 O
Similiar observation has been reported by Teramoto et al., (1969).
(E) Sodium chlorite
The absorption of NO in aqueous mixed solutions of NaClO 2 and
NaOH has been studied by Teramoto et al., (1976) and Sada et al.,
(1978, 1979). Oxidation of NO with NaClO 2 in the presence of NaOH
can be presented by
4 NO + 3 Cl0 2 4 OH ----> 4 NO 3 + 3 Cl - + 2 H 2 0
The rate constant decreases with an increase in the concentration
of NaOH.
(F) Chlorine dioxide
Two Japanese patents claim the use of chlorine dioxide for
NO oxidation (Kushiro, et al., 1976 and Hayashi et al., 1976) The
process was applied the removal of NO from flue gas. The
conversion with respect to NO has been claimed to be about 90%.
(G) Sodium sulfite
Takeuchi, et al., (1977a, 1977b, 1978) have studied the
absorption of NO in aqueous solutions of sodium sulfite at 25 °C.
The absorption of NO in Na 2SO 3 solution was accompanied by a
fast pseudo second order reaction with respect to NO and zero
30
order in sulfite.
Uchida, et al., (1983) studied NO absorption which initially
increases with Na 2 SO 3 concentration, attains a maxima when
[Na 2 SO 3 ] is 0.1 k mole/m3 and then decreases.
(H) Sodium dithionite
Lahari (1981) measured specific rates of absorption of
NO in aqueous caustic solutions of sodium dithionite (Na 2S2O4)
and sodium sulfide (Na 2 S) at 31 °C and 27 °C respectly. The
reaction between NO and aqueous caustic solution of Na 2 S 2 O 4 was
found to be first order with respect to NO only. The first order
rate constant was found to be 2.3*10 3 s-1 at 31 °C. The
reaction between NO and the caustic aqueous solution of sodium
sulfide was found to be zero order in Na 2S and zero order in NO.
The rate constant was found to be 1.69*10 -4 kgmole/m3 s at 27 °C.
3-3 Absorption of NO followed by complex reaction
(A) FeSO 4
Kustin, et al., (1966) have reported that the NO absorption
into FeSO 4 solution is an equilibrium reaction
NO + FeSO4 <----> Fe(NO)SO 4
The forward reaction was found to be first order in both NO
and FeSO4. The reverse reaction was found to be first order with
respect to the complex Fe(NO)SO 4 . Similar observations have been
reported by Hikita, et al., (1977). The value of second order
31
rate constant for the forward reaction was given by the following
equation:
1337log (k2 ) NO = 10.303 + 0.124 I
T
Where I is the ionic strength.
(B) Fe3 -EDTA
The absorption of NO in aqueous solution of Fe 3 -EDTA and
aqueous slurries of MgSO 3 with Fe2-EDTA was carried out by Sada,
et al., (1981). The the forward complexing reaction was
represented by:
NO + Fe 3 -EDTA ----> Fe 3 -(NO)(EDTA)
The reaction was found to be first order in NO and 1.5 order
in Fe 3 -EDTA at pH of 7.0.
(C) Na 2S03/Fe 3 -EDTA-Na
Hikita, et al., (1978) performed a series of experiments for
the NO absorption into Na2SO3/Fe3-EDTA-Na solution. The rate
constants are independent of Na2SO3 concentration and dependent
on Fe 3 -EDTA-Na, ionic strength of present and the temperature.
(D) Ferrous cysteine
The absorption of NO in Ferrous cysteine aqueous solution was
reported by Shih-Ger Chang et al., (1988). The advantage over the
conventional Fe+2-EDTA system is cysteine can be regenerated. The
best regeneration result were obtained from the reaction of CYS-
CYS with H 2S and SO 2 at pH of 10.0 and 60 0C.
32
4 EXPERIMENTAL PROGRAM
The laboratory apparatus, chemicals and gases used to
conduct the experimental program of this thesis are described in
this section.
4-1 Apparatus
The basic scrubbing apparatus consists of a flue gas
blending system, a matched pair of gas scrubbers and an
analytical train. The blending system is capable of producing a
mixture of nitrogen, oxygen, nitric oxide and nitrogen dioxide in
order to simulate a wide variety of flue gas compositions by
mixing pure gases components and blends in different proportions.
The synthetic flue gas was introduced into the scrubber through a
polyethylene sparger. The effluent gases were carried to the gas
analyzers for the determination of residual NO, NO 2 and 0 2 .
The scrubber consists of a 5.1 cm diameter by 61 cm long pyrex
pipe capped on both ends with 0.63 cm entrance and exit
polyethylene tubes. The components of the scrubbing system are
described in detail below.
4-1-1 Flue gas blending system
The flue gas blending system is designed to provide
synthetic flue gas from high pressure gas cylinders. The
blending system is constructed out of 0.63 cm diameter (O.D.) by
60 cm long stainless steel type 316 tubing. Each component gas is
33
delivered through a 0.32 cm (O.D.) teflon tubing to a calibrated
rotameter to the blending mainfold. The total flow rate from the
blending system ranged from 8.9 to 0.4 standard liters per
minute. Figure 4 presents the schematic of the gas blending
system.
4 - 1-2 Scrubbing vessels
The flue gas scrubbers are 5.1 cm in diameter by 61 cm long
Pyrex glass columns with a polyethylene sparger located at the
botton. The flue gas was introduced into the scrubbing solution
through the sparger. Figure 5 contains a schematic of the
scrubber design. Two scrubbing units are available, Either one
could be used as the main scrubber while the other would be the
reference scrubber.
One liter of scrubbing solution is generally used to run
these experiments as semibatch process with continuous (1 l/min)
flue gas injection. The actual scrubbing volume was varied as the
flow rate in a few experiments. The exiting gas was carried to an
oxygen monitor and NO x analyzer from the top of the glass column
by 0.32 cm polyethelene tubing after removal of moisture. Liquid
samples were removed periodically through a valve at the botton
of the glass column to measure solution pH. The same solution was
then injected in a Dionex Ion chromatograph to measure the anion
concentrations of reactant (ClO 2 - ) and products (Cl and NO3-).
Figure 4 Schematic of the Gas Blending System
35
When conducting a scrubbing experiment, the following
sequence was followed:
* Set the flue gas composition and flow rate while flowing
through the reference scrubber.
* Switch flue gas through the main scrubber (prefilled with
the desire solution).
* Monitor pH and temperature.
* Check inlet flue gas composition using the oxygen monitor
and NOx analyzer.
* Measure outlet composition as a function of run time.
* Take liquid sample periodically.
* Run liquid sample in ion chromatography.
4-2 Reagents
NaClO 2 and NaOH were used as the NO oxidizer and NO 2
neutralizer in the scrubber. The synthetic flue gas was produced
by mixing NO, NO2 and O2 gases from high pressure gas cylinders
with nitrogen making up the balance. The source and purity of
these chemicals are discussed below.
4-2-1 Sodium chlorite
Commercial sodium chlorite containing approximately 80%
NaClO2, 18% Na t CO 3 and 2% Na 2 SO4 was supplied by Pfaltz & Bauer
inc. Waterburg, CT 06708.
Textone is the commercial name of sodium chlorite, which is
36
prepared by the reaction of a reducing agent on chlorine dioxide
in an alkaline medium.
Sodium chlorite is the sodium salt of the unstable chlorous
acid, having a dissociation constant of K a = 0.01. The salt
exists both in anhydrous and a trihydrated form. The crystals of
NaClO 2 are white, but they usually have a greenish tint because
traces of chlorine dioxide are present. The chlorite anion is
more stable than hypochlorite anion, but less stable than
chlorate anion. In cold alkaline solutions, sodium chlorite has
only mild oxidizing power. However, heating or acidification, or
both, make chlorite solutions strongly oxidizing. Due to the
strong oxidizing risk, sodium chlorite should never be stored in
open containers.
4 - 2 -2 Sodium hydroxide
Commercial sodium hydroxide (caustic soda, sodium hydride,
lye, white caustic) NaOH was supplied by Pfaltz & Bauer, Inc. CT.
06708. It is a white deliquescent solid, absorbs water and carbon
dioxide from the air. Its melting point is 318 °C, boiling pointis 1390 °C. At normal condition, it disolves in water, alcohol,and glycerol.
Sodium hydroxide is corrosive to tissue in presence of
moisture: strong irritant to tissue (eyes, skin, mucous
membranes). Its tolerance amount is 2 mg per cubic meter of air.
4 - 2 -3 Gases
37
A) Nitric Oxide:
The nitric oxide was supplied as a 10% blend by
Union Carbide Corporation
Linde Division
National Specialty Gases Office
100 Davidson avenue
Somerset, NJ 08873
By the following order informations and specifications
Cylinder type: T
Grade: custom blend
Concentration: 10% NO in nitrogen
Volume: 6.79 m3
Cylinder pressure: 14,824 kpa.
Regulator used: UPG-CGA 160/660 in stainless steel
Nitric oxide is a colorless gas, which is slightly soluble in
water and noncombustible. It usually used to prepare nitrosyl
carbonyl.
It is highly toxic by inhalation; strong irritant to skin and
mucous membrances; supports combustion. Its tolerance
concentration is 25 ppm in air.
B) Nitrogen Dioxide:
The nitrogen dioxide mixed cylinder was supplied as a 0.5%
blend by Union carbide corporation, Linde Division. by the
following order informations and specifications:
Cylinder type: T
38
Grade: custom blend
Concentration: 0.5% NO 2 in Nitrogen.
Volume: 6.79 m3
Cylinder pressure: 14,824 kpa
Regulator used: UPG-CGA 160/660, stainless steel
Nitrogen dioxide is a red-brown gas or yellow liquid; becomes
colorless solid at -11.2 °C, which exists in varying equilibrium
with other oxides of nitrogen as the temperature is varied.
Nitrogen dioxide is used in the production of nitric acid,
nitrating agent, oxidizing agent, catalyst, oxidizer for rocket
fuel, and polymerization inhibitor for acrylates.
NO2 has been found to be very toxic, inhalation may be fatal.
Its tolerance concentration is 5 ppm.
C) Oxygen:
The pure oxygen cylinder was supplied by Union Carbide
Corporation, Linde Division by the following order informations
and specifications
Cylinder type: T.
Grade: extra dry.
Minimum purity: 99.6%
Maximum moisture 10 ppm
Volume: 7.05 m 3
Weight: 69.5 kg
Cylinder pressure: 15,168 kpa
Regulator used: UPE-CGA 540 brass
39
Oxygen is a colorless, odorless, tasteless diatomic gas. It
is actually a noncombustion gas, but actively supports combustion
and may explode on contect with heat oxidizable material. High
concentration oxygen also irritant to skin and tissue.
D) Nitrogen:
The nitrogen gas cylinder was supplied by Union Carbide
Corporation, Linde Division. by the following order informations
and specifications
Cylinder type: T
Grade: prepurified
Minimum purity: 99.998%
H2O < 3 ppm
02 < 5 ppm
volume: 6.46 m3
Weight: 67.7 kg
Cylinder pressure: 15,168 kpa
Regulator used: UPE-CGA 580 brass
Nitrogen is a colorless, tasteless diatomic gas consitituting
about four fifths of the air. The gas is noncombustible and
slightly soluble in water and alcohol.
4-3 Analysis
Concentration changes were obtained by a train of analytical
instruments. They are
* Chemiluminescent NO/NOx analyzer
Model 10 A
Thermoelectron Instruments Inc.
* Beckman NO/NOx analyzer
Model 955
Beckman Industrial Corp.
* Process oxygen monitor
Model 715
Beckman Industrial Corp.
* Beckman oxygen monitor
Model 7005
Beckman Industrial Corp.
* pH meter
Model 6071
Jenco Electronic LTD.
* Ion chromatography
System 2120i
Dionex Corporation
4-3-1 Flue gas analysis
The outlet line carrying the scrubber effluent entered a
junction from which two lines exited. These lines deliver the
effluent to the analyzers for measurement of the concentrations
of NO, NO 2 and O 2 .
The Thermoelectron NOx analyzer is used to measure the
41
concentration of NO and NO 2 . This instrument make use of the
chemiluminescent properties of the NO reaction with ozone as a
measure of NO concentration. NO molecules are mixed with O 3 from
the ozone generator and carried into a reaction chamber where NO
is electronically converted to excited NO 2 . Excited NO 2 than emit
light (Chemiluminescence).
NO + O3 ----> NO 2* + O 2
NO 2* ----> NO 2 hv
The basic chemiluminescent analyzer is only sensitive to NO
molecules. NO 2 is measured by thermally converting the NO 2 to NO
and measuring the incremental increase in concentration of NO.
A temperature controller is used to maintain the proper
temperature for NO 2 reduction. The proper temperature is 650 °C.
Both concentrations of NO and NO 2 were analyzed by a Beckman
955 NO/NOx analyzer independently to double check the results.
Both instruments were calibrated with a analyzed mixture of
100 ppm NO and 109 ppm NO 2 in nitrogen. In all runs, the
agreement between both measurements are better than 10%. The
output signal from both analyzers were continuously recorded.
The oxygen concentration is determined with a Beckman model
715 process oxygen monitor. This instrument has an amperometric
oxygen sensor, which contains a metallic cathode and anode. The
two electrodes are separately mounted within the polypropylene
body and are electrically connected by a potassium chloride
42
electrolyte. A constant potential is impressed across the two
electrodes.
A gas-permeable teflon membrane separates the electrodes
from the process sample and fits firmly against the cathode.
Oxygen from the sample diffuses through the membrance and is
reduced at the cathode. The resultant electrical current flow
between the anode and cathod is proportional to the partial
pressure of oxygen in the sample.
The sensor is calibrated with an analyzed 5% oxygen in
nitrogen mixture and an air sample assumed to be 21% oxygen.
Signal from the monitor is recorded continuously.
A Beckman model 7005 oxygen monitor is used to double check
the result from model 715. The agreement between these two
instruments is always better than 10%.
4-3-2 Scrubbing solution analysis
A Jenco 6071 pH meter was used to measure the pH value and
temperature of the scrubbing solution. Janco 6071 is a
microcomputer based pH/mv/temperature meter with following
specifications:
* Range
pH: -2.00 to 16.00
Temperature: 0 to 100 °CmV: -999 to 999
* Resolution
pH: 0.01
Temperature: 0.1
mV: 1.0
* Accuracy
pH: 0.1%
Temperature: 0.5%
mV: 0.1%
Liquid samples were taken from the scrubber at regular time
interval. Ionic concentrations were determined both qualitatively
and quantitatively using Dionex ion chromatography system 21201
under the following conditions:
Column: BIO-RAD
IC-anion-PW
Catalogue-127-0062
Serio # 10825
Flow rate: 1.2 ml/min
Low pressure limit: 0 psi
High pressure limit: 700 psi
Temperature compensation: 1.7
Output range: 30 us
The sample injection is pushed by buffer eluent through the
analytical column, where ions are separated. After separation,
the sample is carried to a conductivity detector, which is based
upon the electrical conductivity of the sample when placed
between two oppositely charge electrodes. The presence of ions in
44
the solution allows electrical current to flow between the
electrodes, completing the circuit. At low concentrations,
conductivity is directly proportional to the concentration of
conductive species in the solution. The total ionic concentration
of the solution in the cell and the temperature of that solution
affect the linearity of this relationship.
Table 7 contains data on a series of standard at varies
concentrations. Figure 6 is the peak area-concentration
calibration curve of Cl - and NO3 - , using this curve,
concentrations of Cl - and NO 3 - can be obtained.
Table 7 Peak Area-Concentration Data of Cl - and NO 3 - of
Dionex IC Systen 2120i
conc.
ppm.
species
anion
peak area
10 7
400Cl
NO3_
26.52
14.65
200Cl
NO3_
15.05
7.08
100Cl
_NO3
7.33
3.52
50Cl -
NO3 _
3.20
1.61
25Cl -
NO3
1.58
0.80
12.5Cl -
_NO3
0.69
0.25
Figure 6 Relationship Between Peak Area and Concentration of
Cl- and NO 3- of Dionex IC System 2120i
5 RESULTS
An aqueous scrubbing program was conducted to test sodium
chlorite as an in situ oxidizer to absorb NO by means of
oxidation and absorption in an alkaline solution from a gas
stream that simulates the composition of incineration emission.
The objective of the scrubbing tests were to:
* Determine the potential of NaClO 2 aqueous scrubbing for NO
oxidation and absorption.
* Obtain the rate parameters for NO oxidation to NO 2 by
NaC1O 2 .
Experiments are detailed in the following sections.
5-1 Absorption of NO in Caustic Aqueous Scrubbing
A series of runs of aqueous scrubing of flue gas containing
NO of 300, 500, and 800 ppm in N 2 had been performed at following
conditions
Flue gas:
NO: 300, 500, 800 ppm
NBalance:• 2
Flow rate: 1 l/min
Scrubbing solutions:
NaOH: 0, 0.0001, 0.01, 1.0 M
Volume: 1 liter
All tests were conducted at room temperature (25 °C) andatmospheric pressure.
Results contained in Table 8 indicate that NO can not be
absorbed by distilled water. Similarly, the presence of NaOH up
to 1.0 M did not improve the NO absorption.
Table 8 Absorption of NO in Caustic Aqueous Scrubbing
absorption %NO (ppm) Expt
#
Conc.
0
of NaOH
0.0001
(molarity)
0.1 1
300 1 0.3 0.8 0.7 0.6
2 0.4 1.3 1.1 1.2
3 0.7 1.1 0.3 1.3
avg 0.5 1.1 0.7 1.0
500 1 0.8 1.8 1.7 0.7
2 0.3 0.9 0.9 0.8
3 1.1 0.6 0.3 1.0
avg 0.7 1.1 1.0 0.8
800 1 0.7 2.1 0.6 0.7
2 0.4 1.1 0.0 1.4
3 0.6 0.4 1.7 1.2
avg 0.6 1.2 0.8 1.1
NOAbsorption % = 1 - * 100%
NO (in)
50
5-2 Absorption of NO 2 in Caustic Aqueous Scrubbing
A number of experiments were carried out to test the
absorption of NO 2 in caustic aqueous scrubbing. which were
conducted at the following conditions
Flue gas:
NO2 : 300, 500, 800 ppm
Balance: N2
Flow rate: 1 1/min
Scrubbing solutions:
NaOH: 0.01, 0.1, 0.5 M
Volume: 1 liter
The experiments were conducted at room temperature and
pressure.
Data in Table 9 show that NO 2 can be effectively absorbed by
NaOH solution. Runs with 0.5 M NaOH indicated that absorption of
more than 98% is achieved.
Table 9 Absorption of NO 2 in Caustic Aqueous Scrubbing
NO 2 Expt
(ppm) #
absorption %
Conc. of NaOH (molarity)
0.01 0.1 0.5
300 1 74.0 76.4 95.9
2 73.3 72.5 96.6
3 72.8 72.4 96.2
avg 73.4 73.8 96.6
500 1 80.0 84.4 97.2
2 80.8 81.8 97.4
3 80.0 82.8 98.2
avg 80.3 83.0 97.6
800 1 83.4 90.0 97.5
2 81.4 89.1 98.6
3 80.6 89.1 98.1
avg 81.8 89.9 98.1
NO 2 (out)Absorption % = 1 - * 100%
NO2 (in)
52
5-3 NO2 effect on NO absorption in caustic aqueous
scrubbing
A number of experiments were performed to evaluate the
effect of the reaction
NO + NO2 <----> N 2O 3
N2O3 + H2O ----> 2 HNO 2
in caustic aqueous scrubbing. Experiments were conducted at the
following conditions
Flue gas:
Equal molar of NO and NO2 at 300, 500, 800 ppm
NBalance: 2
Flow rate: 1 1/min
Scrubbing Solutions
NaOH: 0.01, 0.5 M
Volume: 1 liter
There tests were conducted at room temperature and
atmospheric pressure.
The actual measurements are summarized in Tables 10 and 11.
These data indicate that the presence NO2 significantly increases
NO absorption in caustic aqueous solution.
Table 10 NO2 Effect on NO Caustic Aqueous Scrubbing
Conc.(ppm)
Expt # NO(in)(ppm)
NO(out)(ppm)
Abs%
NO 2 (in)(PPm)
NO 2 (out)(PPm)
Abs
300 1 361 136 62.3 318 5 98.4
2 330 84 74.5 295 11 96.3
3 325 125 61.5 352 13 96.3
avg 66.1 97.0
500 1 568 186 67.3 568 23 96.0
2 591 182 69.2 568 25 95.6
3 542 167 69.2 573 22 96.2
avg 68.6 95.9
800 1 813 333 59.0 833 17 98.0
2 843 354 58.0 823 10 98.8
3 813 385 52.6 875 0 100.0
avg 56.5 98.9
* NaOH = 0.01 M
NO* Abs(NO) = 1 * 100%
NO (in)
Table 11 NO 2
Solution
Effect on NO Absorption in Caustic Aqueous
Conc.ppm
Expt # NO(in)ppm
NO (out)ppm
Abs%
NO2 (in)ppm
NO 2 (out)ppm
Abs%
300 1 310 5 98.5 324 0 100
2 355 19 94.6 324 0 100
3 379 17 95.6 336 0 100
avg 96.2 100
500 1 576 21 96.3 602 0 100
2 595 14 97.6 602 0 100
3 600 17 97.3 595 0 100
avg 97.0 100
800 1 845 21 97.5 840 0 100
2 867 24 97.3 893 0 100
3 826 10 98.8 833 0 100
avg 97.9 100
* NaOH = 0.5 M
5-4 Oxygen Effect on NO Absorption in Caustic Aqueous
Scrubbing
Oxygen is generally present in incinerator emissions because
excess air is used to make sure incinerator feed is completely
combusted. Since oxygen react with NO to form NO 2 , it is
important to determine its contribution to NO absorption.' the
following runs were performed to test this effect.
Flue gas
NO: 300, 500, 800, 2000 ppm
O2 : 5%
Balance: N 2
Flow rate: 1 1/min
Scrubbing solutions
NaOH: 0.0001, 0.01, 1 M
Volume: 1 liter
All runs were conducted at room temperature and pressure.
Results in Table 12 indicate that no significent change in NO
absorption in caustic aqueous scrubbing occurs due to
introduction of 5% O 2 into synthetic flue gas.
Table 12 Effect of 5% Oxygen on NO Absorption in Caustic
Aqueous Scrubbing
NO(ppm) Expt # absorption %
Conc. of NaOH (molarity)
0.0001 0.01 1.00
300 1 0.0 0.5 0.0
2 1.8 0.0 1.2
3 1.1 0.8 0.3
avg 1.0 0.4 0.5
500 1 1.5 0.2 0.2
2 1.9 1.1 0.1
3 0.0 0.8 1.7
avg 1.1 0.7 0.7
800 1 1.4 0.0 2.0
2 0.0 1.4 0.9
3 0.0 1.4 0.0
avg 0.4 0.9 1.0
2000 1 1.3 0.5 1.7
2 0.0 1.0 1.0
3 0.4 0.4 4.5
avg 0.6 0.6 2.4
NOAbsorption % = 1 * 100%
NO (in)
57
5-5 Oxidation of NO by NaClO2 in aqueous scrubbing
A number of experiments were performed to evaluate the
potential of sodium chlorite (NaClO2) to oxidize NO in aqueous
scrubbing under the following conditions
Flue gas
NO: 2000 ppm
Balance: N2
Flow rate: 1/min
Scrubbing solution
NaClO2: 0.008 M
Na2 SO4: 1.3*10-4 M
Na2 CO3: 1.5*10-3 M
Volume: 1 liter
These runs were conducted at room temperature and pressure.
The first set of experiments was made to evaluate the
oxidation and absorption of NO in NaClO 2 aqueous scrubbing and to
ascertain the products and material balance in this reaction. Due
to the impurities (18% Na 2 CO 3 and 2% Na 2 SO4 ) in NaClO 2 , the
scrubbing solution had an initial pH of 10.0.
The actual measurements listed in Table 13 show that NO is
quantitatively oxidized to NO 3 - in the liquid during the
scrubbing. Hydronium ion was produced to decrease pH to 3 after
the first few minutes and 1.8 at the end point of the experiment
(Table 14, Figure 7). The end point is reached when NO breaks
through.
58
Table 13 NO Oxidation by NaClO2 in Aqueous Scrubbing
Phase Item Expt #1 Expt #2 Expt #3
Gas Conc. of NO 2,325 ppm 2,075 ppm 2,175 ppm
Flow rate 1 1/min 1 1/min 1 1/min
Time (B.K.) 80 min 89 min 88 min
Moles 0.00759 0.00754 0.00781
liquid Conc. of NaClO 2 0.008 M 0.008 M 0.008 M
Conc. of Na 2CO 3 0.0015 M 0.0015 M 0.0015 M
Conc. of Na 2SO 4 0.00038 M 0.00038 M 0.00038 E
Volume 1 liter 1 liter 1 liter
pH before 10.1 10.l 10.1
pH after 1.82 1.99 1.83
Conc. of NO 3- 480 ppm 460 ppm 480 ppm
Mole of Cl 0.0080 0.0082 0.0083
Mole of NO 3 0.0077 0.0074 0.0077
NO/ NO 3 0.98 1.02 1.01
ClO 2 Cl 1.00 1.03 1.04
NO/C1O 2 - 0.95 0.94 0.97
Time (B.K.): Break through time
Effectiveness: NO/ClO2 = 0.95
Table 14 pH Change in NO Scrubbing with 80% NaClO2 as
Sample
Received
# Time (min) Expt #1 Expt #2 Expt #3
1 0 9.87 9.77 10.10
2 2 7.53 7.31 8.59
3 10 3.49 3.70 3.65
4 20 3.06 3.06 3.01
5 30 2.71 2.80 2.68
6 40 2.57 2.58 2.43
7 50 2.54 2.43 2.38
8 60 2.23 2.20 2.13
9 70 2.16 2.21 2.06
10 80 1.98 2.14 1.96
11 90 1.98 1.98 1.98
12 100 1.93 1.93 1.86
Figure 7 pH Change in NO Aqueous Scrubbing
61
The second set of experiments were performed to figure out
the products formation during the experiment. Experiments were
conducted at the following conditions:
Flue gas
NO: 2,000 ppm
Balance: N2
Flow rate: 1 1/min
Scrubbing solution
NaClO 2 : 0.008 M
Na2CO3: 0.0015 M
Na2SO4: 0.00038 M
Volume: 1 liter
Room temperature and atmospheric pressure
Samples were taken out at fixed time interval to determine
Cl - and NO 3 - concentrations by Ion chromatography. Figure 8;
derived from Table 15, indicates that the formation rates of both
products are constants as the concentration of NaClO2 changes
from 0.008 to 0 Molar. This data indicates that mass transfer may
be limiting the process.
62
Table 15 Concentrations of NO 3 - and Cl - with Respect to
Reaction time
Sample # Time RT(1) PA (2) Conc.(ppm) Conc.(M)
(min) (min) 10 7 10-2
1 0 3.09 3.33 92 0.26
2 3 3.13 3.74 108 0.30
5.51 0.38 28 0.05
3 20 3.13 5.98 164 0.46
5.51 2.49 140 0.23
4 40 3.18 8.78 240 0.68
5.43 5.28 284 0.46
5 52 3.21 10.41 290 0.82
5.41 7.70 420 0.68
6 61 3.20 10.72 300 0.85
5.40 7.98 440 0.71
7 70 3.23 10.70 300 0.85
5.40' 8.51 490 0.79
8 75 3.22 10.69 300 0.85
5.41 8.54 492 0.79
(1) RT: Average retention time
RT of Cl - is 3.10 to 3.20 minute
RT of CO 3 5.40 to 5.50 minute
(2) PA.: Average Peak area
63
Figure 8 Concentration-Time Curves of NO Scrubbing
64
5-6 Oxidation of NO by NaClO2 in Caustic Aqueous
Scrubbing
In order to compare the NO absorption effectiveness of
caustic scrubbing to neutral solution, several runs were made
with NaOH under the following conditions
Flue gas
NO: 2,000 ppm
Balance: N 2
Flow rate: 1 1/min
Scrubbing solution
NaClO 2 : 0.01 M
Na2CO3: 0.0015 M
Na2SO4: 0.00038 M
NaOH: 0.001 M
Volume: 1 liter
All experiments were conducted at room temperature and
pressure.
Result containing in Table 16 indicate that the effectiveness
of NO absorption in NaClO 2 aqueous scrubbing was slightly
improved (NO/ClO 2 - rise to l.04) by NaOH addition.
Table 16 Effectiveness of NO Absorption in NaClO2 Caustic
Aqueous Scrubbing
Phase Item Expt #1 Expt #2 Expt #3
Gas Conc. of NO 1,950 ppm 1,925 ppm 2,075 p :
Flow rate 1 1/min 1 1/min .1 1/min
Time (B.T.) 104 min 105 min 100 min
moles of NO 0.0083 0.0083 0.0085
Liquid Conc of NaClO 2 0.0080 0.0080 0.0080
Conc of Na 2 CO 3 0.0015 0.0015 0.0015
Conc of Na 2 SO4 0.00038 0.00038 0.00038
conc. of NaOH 0.00097 0.00097 0.00097
Volume 1 liter 1 liter 1 liter
pH before 11.27 11.27 11.27
pH after 1.69 1.75 1.73
Conc. of NO 3_
510 ppm 520 ppm 510 ppm
Mole of Cl 0.0083 0.0086 0.0087
Mole of NO 3 0.00823 0.00839 0.00823
NO/NO 3 1.01 0.99 1.03
Cl/ ClO 2 1.03 1.05 1.03
NO/ ClO 2 1.04 1.04 1.06
5-7 Oxygen Effect on NO Absorption in NaClO2 Aqueous
Scrubbing
When flue gas containing O 2 is bubbled through a NaClO 2
containing scrubber, NaClO 2 may react with 0 2 . In order to
ascertain this effect, several runs were made at the following
conditions
Flue gas
NO: 2,000 ppm
O 2 : 5%
Balance: N 2
Flow rate: 1 1/min
Scrubbing solution
NaClO 2 : 0.01 M
Na2CO3: 0.0015 M
Na2SO4: 0.00038 M
Volume: 1 liter
All experiments were conducted at room temperature and
pressure.
Table 17 shows that the effectiveness of NO scrubbing by
NaClO 2 decreased (NO/ClO 2 - drop to 0.84) when 0 2 was present in
the flue gas.
67
Table 17 Oxygen Effect on NO Absorption in NaClO2 Scrubbing
Phase Item Expt #1 Expt #2 Expt #3
Gas Conc. of NO 1,883 ppm 1,841 ppm 1,883 ppm
Flow rate 1 1/min 1 1/min 1 1/min
Break through 84 min 90 min 90 min
Mole of NO 0,0065 0.0068 0.0069
Liquid Conc of NaClO 2 0.008 0.008 0.008
Conc of Na 2 CO 3 0.0015 0.0015 0.0015
Conc of Na 2 SO 4 0.00038 0.00038 0.00038
volume 1 liter 1 liter 1 liter
_mole of ClO 2 0.008 0.008 0.008
NO/ ClO 2 - 0.81 0.85 0.86
5-8 pH Effect on NO Absorption in NaClO2 Aqueous
Scrubbing
Based on the results reported in section 5-6, a certain
amount of NO breaks through with NaClO 2 scrubbing in presence of
0.0OlM NaOH. It was presumed that NaOH may inhibit or may lower
the mass transfer coefficient of this process. In order to
ascertain the veracity of this prediction, a number of
experiments were performed under the following conditions
Flue gas
NO: 2,000 ppm
Balance: N 2
Flow rate: 1 1/min
Scrubbing solution
NaClO 2 : 0.00008, 0.0008, 0.008, 0.08 M
Na2CO3: 0.000015, 0.00015, 0.0015 and 0.015 M
Na2SO4: 0.0000038, 0.000038, 0.00038 and 0.0038 M
NaOH: 0.0001, 0.001, 0.005, 0.01, 0.l, 1 M
Volume: 1 liter
All runs were conducted at room temperature and pressure.
Figure 9, derived from Table 18, indicate that high
concentration of NaOH inhibit NO absorption in NaClO 2 aqueous
scrubbing.
Table 18 pH Effect on NO Absorption in NaClO 2 Caustic Aqueous
Scrubbing
Conc. of NaOH Expt # Per-cent Absorption
(M) 0.00008
Conc. of NaClO 2
0.0008 0.008 0.08
0.0001 1 1 0 100 1002 2 0 100 1003 1 0 100 100avg 1 0 100 100
0.001 1 1 0 100 1002 0 1 100 1003 3 0 100 100avg 1 0 100 100
0.005 1 - - 25 1002 - - 24 1003 - - 25 100avg 25 100
0.01 1 0 0 4 1002 0 2 6 1003 0 1 8 100avg 0 1 6 100
0.1 1 1 0 0 172 0 1 0 173 1 1 1 19avg 1 1 0 18
1.0 1 0 2 0 02 0 2 1 23 4 1 0 0avg 1 2 0 1
NOAbsorption % = 1 * 100%
NO (in)
Figure 9 PH Effect on NO Absorption in NaClO 2 Caustic
Aqueous Scrubbing
6 DISCUSSION
A semibatch reactor was used to evaluate the potential of
oxidizing NO with NaClO 2 in an alkaline scrubbing solution. The
synthetic flue gas was blended from compressed gas cylinders and
was continuously fed to the scrubbing solution which was the
stationary phase in the reactor. The flue gas was introduced
into the reactor through a polyethylene sparger located at the
botton of the reactor. Thus the bubble size was influenced by
the pressure of flue gas, the size the sparger openings, and
the viscosity of the scrubbing solution.
The absorption of NO by an aqueous solution of alkaline
NaClO 2 was accomplished at room temperature and 101.3 kPa of flue
gas pressure. A greenish yellow color in the solution was found
after three minutes of operations and disappeared after 105
minutes. It is assumed that the color represents an intermediate
in the absorption of NO. The mole ratio of the total amount of NO
that flowed through the reactor to NaClO 2 is 0.95 at
breakthrough. It should be reemphasized that 80% NaClO 2 was used
with the rest of the starting material in solution consisting of
18% Na 2 CO3 and 2% Na 2 SO4. Thus, the initial composition of the
scrubber liquid was quite alkaline. Under all conditions when
NaClO2 was present, the other two reagents were also present. For
expediency, this fact may not be mentioned every time NaClO 2
chemistry is discussed.
Low concentrations of NaOH increase the NO removal
72
effectiveness. High concentrations of NaOH inhibited the
absorption. The upper limit for NaOH concentration necessary to
completely absorb NO in NaClO 2 aqueous solution depends on the
concentration of NaClO 2 . NO was absorbed quantitatively in 0.008
molar NaClO2 aqueous solution when the concentration of NaOH was
equal to or lower than 0.001 molar. The upper limit of NaOH in
0.08 molar NaClO 2 aqueous solution is 0.1 molar. At the upper
limit concentration of NaOH, absorption can not reach 100% within
the first few minutes. If the NaOH concentration is a little
higher than upper limit, absorption goes to zero after a few
minutes of running time.
NaOH in NaClO2 aqueous solution increases the effectiveness
of NO to NaClO2 reactant ratio at breakthrough to 0.96 - 1.05
while 5% oxygen in the flue gas decreases it to 0.84.
Before saturation, the concentration-time curves are straight
lines of different slopes. Slope of NO 3 - is greater than that
of C1 - . The rate of formation of NO 3 - is faster than that of C1.
The time between their formation is the life time of the green
intermediate.
The pH value of the aqueous solution drops drastically from
10 to 3 within the first ten minutes. The solution was prepared
with 0.008 molar of NaClO 2 and 0.0015 molar of Na 2CO3 owing to
the 18% (w/w) and 2% Na 2SO4 impurity. The pH value of this
solution is 10. After three minutes running, the reaction was
carried out under acidic condition.
After NaClO2 in scrubbing solution was used up, only two
products were found in the scrubbing solution. They were
NO3-and Cl. The CO3-2converted to H2CO3when pH fell below 3. The
pH value of the scrubbing solution is 1.9. Under this situation,
no CO 3 -2 exisit in the solution. Since no CO 3 -2 signal is found
in ion chromatography.
74
7 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORKS
7-1 Conclusions
A series of experiments were performed in order to evaluate
the capability of alkaline NaClO2 aqueous solution to scrubb NO
by oxidizing it from synthetic flue gas. In some runs, NO 2 or O 2
were mixed into the synthetic flue gas to simulate the flue gas
emitted by municipal incinerator or stationary power plant. The
conclusions from this research are listed as below:
* At room temperature and atmospheric pressure, NO is not
absorbed by distilled water. NaOH up to 1.0 molar does not
improve this absorption.
* At room temperature and atmospheric pressure, NO 2 can be
absorbed effectively by NaOH aqueous solution. A 0.5 molar
NaOH aqueous solution can scrubb 98% of NO 2 from flue gas
at relatively low NO 2 concentration (lower than 800 ppm).
* The effectiveness of NO absorption in caustic aqueous
scrubbing is greatly increased by the presence of NO 2 in
the flue gas.
* 50 O2 in flue gas does not affect NO absorption in caustic
(NaOH up to 1.0 molar) aqueous scrubbing.
* NO can be oxidized effectively by NaClO 2 in aqueous
solution. During scrubbing, NO is oxidized to NO 3 - , and
ClO 2 - converted to Cl - in scrubbing solution.
* Owing to the production of HNO 3 , pH drops sharply from 10
to 3 within ten minutes in the scrubbing of NO by alkaline
NaClO 2 aqueous solution.
* Rates of Cl - and NO 3 - formation in scrubbing solution are
constants at the low concentration scrubbing process as the
concentration of NaClO2 changes from 0.008 molar to 0
molar.
* Low concentration of NaOH increase the effectiveness of NO
absorption in NaClO 2 aqueous scrubbing solution by 7%,
while concentrations higher than upper limit decreases or
inhibits its absorption.
* Effectiveness (NO/ClO 2 - ratio) decrease from 0.95 to 0.84
when 5% O 2 in presence of flue gas.
7-2 Recommendations for future works
The results of this program clearly indicates the utility
of NaClO 2 for NO x emission control. The best conditions of
chemical reaction of NO x in NaClO 2 and aqueous scrubbing
can be reached from the results presented in this thesis.
However, to develop the engineering data that is essential for
scale-up, pilot plant studies are needed . Some critical factors
which need to be studied in pilot plant process are listed below:
* Rate of NO absorption in alkaline NaClO 2 aqueous solution
in pilot plant conditions.
* The overall mass transfer coefficient and interfacial area
76
of pilot plant process.
* How does temperature affect this process in pilot plant
conditions
* How do other flue gas components affect the NO absorption
in NaClO2 aqueous scrubbing.
* Optimization should be assessed.
* How to treat the scrubbing solution (pH of 1.8 and NO 3 - ).
The purposes of this progrem should be to completely remove
NOx from flue gas and the effect of scrubbing solution must be
minimized to meet the regulations at the same time.
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